Astronomical Spectroscopy - Physics - University of Cincinnati

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$1 Ch. 22 â Reflection and Refraction of Light 22.1 The ... - Physics$

– 38 – Fig. 12.— Normalization of Flats. Left. The flat field counts varies by a factor of of 10 or more with wavelength, and there is a discontinuity in the derivative around pixel 820. Middle. Normalizing the flat by a constant yields a smooth stellar spectrum. Right. Normalizing the flat by a higher order function results in an artificially large gradient in the counts. these, one can combine the exposures (scaling each by the average counts), averaging the spatial profile over all wavelengths, and then fitting a smooth function to the spatial profile. The result is an image that is uniform along each row of the dispersion axis, and hopefully matches the night sky perfectly in the spatial direction. Sadly this is usually not the case, as during bright twilight there is light bouncing off of all sorts of surfaces in the dome. Thus one might need to make a further correction using exposures of the dark night sky itself. This process in illustrated in Figure 13. The relevant IRAF tasks involved are imcombine, illumination, and ccdproc This concludes the basic CCD reductions, and one is now ready to move on to extracting and calibrating the spectrum itself. 8. Identification of Object and Sky. At this stage one needs to understand the location of the stellar spectrum on the detector, decide how wide an extraction window to use, and to select where the sky regions should be relative to the stellar spectrum. The location of the star on the detector will doubtless be a mild function of wavelength, due either to slight non-parallelism in how the grating is mounted or simply atmospheric differential refraction. This map of the location of the stellar spectrum with wavelength is often referred to as the “trace”. Examples are shown in Figure 14. In IRAF this is done either as part of apall or via doslit. 9. Wavelength calibration. Having established the aperture and trace of the stellar spectrum, it behooves one to extract the comparison spectrum in exactly the same manner (i.e., using the same trace), and then identify the various lines and perform a smooth fit to the wavelength as a function of pixel number along the trace. Figure 15
– 39 – Fig. 13.— The spatial profile after flattening by the dome flat (step 6) is shown at the upper left. Clearly there is a gradient from left to right. The profile of the twilight sky is shown in the middle. It’s not a particularly good match, and after dividing by it we see that the situation is not improved (top right). The profile of all of the sky exposures (after division by the twilight profile) is shown at lower left, and was used to re-correct all of the data. After this, the spatial profile was flatter, as shown at lower right, although there is now a modest kink.
Page 1 and 2: Astronomical Spectroscopy Philip Ma
Page 3 and 4: - 3 - Similarly the emphasis here w
Page 5 and 6: - 5 - could see 8000Å light from f
Page 7 and 8: - 7 - ratio of the focal lengths of
Page 9 and 10: - 9 - Fig. 2.— The overlap of var
Page 11 and 12: - 11 - 2.1.2. Choosing a grating Wh
Page 13 and 14: - 13 - 4-Meter Telescope - RC Spect
Page 15 and 16: - 15 - gratings blazed at the red.
Page 17 and 18: - 17 - each order (Equation 9). Thi
Page 19 and 20: - 19 - wants can be a real challeng
Page 21 and 22: - 21 - Fig. 7.— Multi-object mask
Page 23 and 24: - 23 - Fig. 8.— Optical layout of
Page 25 and 26: - 25 - Fig. 9.— View of the focal
Page 27 and 28: - 27 - Fig. 10.— Optical design o
Page 29 and 30: - 29 - When built twenty years ago,
Page 31 and 32: - 31 - corrected” image cube is c
Page 33 and 34: - 33 - • The data frames themselv
Page 35 and 36: - 35 - Fig. 11.— Three examples o
Page 37: - 37 - bumps and wiggles are presen
Page 41 and 42: - 41 - one might want to do this on
Page 43 and 44: - 43 - The steps involved in this r
Page 45 and 46: - 45 - be obtained for each grating
Page 47 and 48: - 47 - the flat-field division. Alt
Page 49 and 50: - 49 - night. More about this is di
Page 51 and 52: - 51 - can be answered given a comp
Page 53 and 54: - 53 - that it degraded the signal-
Page 55 and 56: - 55 - 3.2.6. Long-Slit Flat-Fieldi
Page 57 and 58: - 57 - over the “perfect” flat-
Page 59 and 60: - 59 - 3.2.7. Radial velocities and
Page 61 and 62: - 61 - A good general line list (id
Page 63 and 64: - 63 - the only overlap would be wi
Page 65 and 66: - 65 - a far better job at matching
Page 67 and 68: - 67 - dispersion. Going to the par
Page 69 and 70: - 69 - red (including at least 7770
Page 71 and 72: - 71 - Exposures of the dome flat q
Page 73 and 74: - 73 - A bright star was seen just
Page 75 and 76: - 75 - 500 stars. Figure 20 shows t
Page 77 and 78: - 77 - So, attempt to select a tell
Page 79 and 80: - 79 - Fig. 21.— Output from the
Page 81 and 82: - 81 - Wrap-Up and Acknowledgements
Page 83 and 84: - 83 - Massey, P., DeVeny, J., Jann

Astronomical Spectroscopy - Physics - University of Cincinnati

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